Conservation experts have already signalled that the world is in the grip of the "sixth great extinction" of species, driven by the destruction of natural habitats, hunting, the spread of alien predators and disease, and climate change.

However until recently it has been hoped that the rate at which new species were evolving could keep pace with the loss of diversity of life.

Speaking in advance of two reports next week on the state of wildlife in Britain and Europe, Simon Stuart, chair of the Species Survival Commission for the International Union for the Conservation of Nature – the body which officially declares species threatened and extinct – said that point had now "almost certainly" been crossed.

"Measuring the rate at which new species evolve is difficult, but there's no question that the current extinction rates are faster than that; I think it's inevitable," said Stuart.

The IUCN created shock waves with its major assessment of the world's biodiversity in 2004, which calculated that the rate of extinction had reached 100-1,000 times that suggested by the fossil records before humans.

No formal calculations have been published since, but conservationists agree the rate of loss has increased since then, and Stuart said it was possible that the dramatic predictions of experts like the renowned Harvard biologist E O Wilson, that the rate of loss could reach 10,000 times the background rate in two decades, could be correct.

"All the evidence is he's right," said Stuart. "Some people claim it already is that ... things can only have deteriorated because of the drivers of the losses, such as habitat loss and climate change, all getting worse. But we haven't measured extinction rates again since 2004 and because our current estimates contain a tenfold range there has to be a very big deterioration or improvement to pick up a change."

Extinction is part of the constant evolution of life, and only 2-4% of the species that have ever lived on Earth are thought to be alive today. However fossil records suggest that for most of the planet's 3.5bn year history the steady rate of loss of species is thought to be about one in every million species each year.

Only 869 extinctions have been formally recorded since 1500, however, because scientists have only "described" nearly 2m of an estimated 5-30m species around the world, and only assessed the conservation status of 3% of those, the global rate of extinction is extrapolated from the rate of loss among species which are known. In this way the IUCN calculated in 2004 that the rate of loss had risen to 100-1,000 per millions species annually – a situation comparable to the five previous "mass extinctions" – the last of which was when the dinosaurs were wiped out about 65m years ago.

Critics, including The Skeptical Environmentalist author, Bjørn Lomborg, have argued that because such figures rely on so many estimates of the number of underlying species and the past rate of extinctions based on fossil records of marine animals, the huge margins for error make these figures too unreliable to form the basis of expensive conservation actions.

However Stuart said that the IUCN figure was likely to be an underestimate of the problem, because scientists are very reluctant to declare species extinct even when they have sometimes not been seen for decades, and because few of the world's plants, fungi and invertebrates have yet been formally recorded and assessed.

The calculated increase in the extinction rate should also be compared to another study of thresholds of resilience for the natural world by Swedish scientists, who warned that anything over 10 times the background rate of extinction – 10 species in every million per year – was above the limit that could be tolerated if the world was to be safe for humans, said Stuart.

"No one's claiming it's as small as 10 times," he said. "There are uncertainties all the way down; the only thing we're certain about is the extent is way beyond what's natural and it's getting worse."

Many more species are "discovered" every year around the world, than are recorded extinct, but these "new" plants and animals are existing species found by humans for the first time, not newly evolved species.

In addition to extinctions, the IUCN has listed 208 species as "possibly extinct", some of which have not been seen for decades. Nearly 17,300 species are considered under threat, some in such small populations that only successful conservation action can stop them from becoming extinct in future. This includes one-in-five mammals assessed, one-in-eight birds, one-in-three amphibians, and one-in-four corals.

Later this year the Convention on Biological Diversity is expected to formally declare that the pledge by world leaders in 2002 to reduce the rate of biodiversity loss by 2010 has not been met, and to agree new, stronger targets.

Despite the worsening problem, and the increasing threat of climate change, experts stress that understanding of the problems which drive plants and animals to extinction has improved greatly, and that targeted conservation can be successful in saving species from likely extinction in the wild.

This year has been declared the International Year of Biodiversity and it is also hoped that a major UN report this summer, on the economics of ecosystems and biodiversity, will encourage governments to devote more funds to conservation.

Professor Norman MacLeod, keeper of palaeontology at the Natural History Museum in London, cautioned that when fossil experts find evidence of a great extinction it can appear in a layer of rock covering perhaps 10,000 years, so they cannot say for sure if there was a sudden crisis or a build up of abnormally high extinction rates over centuries or millennia.

For this reason, the "mathematical artefacts" of extinction estimates were not sufficient to be certain about the current state of extinction, said MacLeod.

"If things aren't falling dead at your feel that doesn't mean you're not in the middle of a big extinction event," he said. "By the same token if the extinctions are and remain relatively modest then the changes, [even] aggregated over many years, are still going to end up a relatively modest extinction event."

Species on the brink of being declared extinct

The International Union for the Conservation of Nature (IUCN) lists 208 species as "possibly extinct", more than half of which are amphibians. They are defined as species which are "on the balance of evidence likely to be extinct, but for which there is a small chance that they may still be extant".

Kouprey (or Grey ox; Bos sauveli)

What: Wild cattle with horns that live in small herds

Domain: Mostly Cambodia; also Laos, Vietnam, Thailand

Population: No first-hand sightings since 1969

Main threats: hunting for meat and trade, livestock diseases and habitat destruction

Webbed-footed coqui (or stream coqui; Eleutherodactylus karlschmidti)

What: Large black frog living in mountain streams

Domain: East and west Puerto Rico

Population: Not seen since 1976

Main threats: Disease (chytridiomycosis), climate change and invasive predators

Golden coqui frog (Eleutherodactylus jasperi)

What: Small orange frog living in forest or open rocky areas

Domain: Sierra de Cayey, Puerto Rico

Population: No sightings since 1981

Main threats: Unknown but suspected habitat destruction, climate change, disease (chytridiomycosis) and invasive predators

Spix's macaw (or little blue macaw; Cyanopsitta spixii)

What: Bright blue birds with long tails and grey/white heads

Domain: Brazil

Population: The last known wild bird disappeared in 2000; there are 78 in captivity

Main threats: Destruction of the birds' favoured Tabebuia caraiba trees for nesting, and trapping

Café marron (Ramosmania rodriguesii)

What: White flowering shrub related to the coffee plant family

Domain: Island of Rodrigues, Republic of Mauritius

Population: A single wild plant is known

Main threats: Habitat loss, introduced grazing animals and alien plants

Source: IUCN and Royal Botanic Gardens Kew. To mark the International Year of Biodiversity, the IUCN is running a daily profile of a threatened species throughout 2010. See iucn.org.

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